It is frequently the case in the chemical, oil and gas industries, and elsewhere, that a gas-phase mixture that includes one or more relatively low-boiling components has to be separated. In principle, many separation techniques are available, including absorption, adsorption, condensation, cryogenic distillation, membrane separation and so on. The optimum technique depends on the specifics of the situation and is influenced by capital outlay, operating costs, energy consumption, physical and chemical properties of the components, value of the components, environmental protection issues, safety and reliability concerns and other factors.
If the boiling points of the components differ, low-temperature condensation and cryogenic distillation are technically possible, but may be impractical, for example because of high refrigeration costs or the need for extensive pre-treatment to remove components that might freeze and plug the system.
Adsorbents and absorbents are component specific and not infrequently are problematic to regenerate or dispose of.
Membrane separation is currently available only for a limited number of gases and may not be able to produce a product of sufficiently high purity.
Thus, although numerous gas separations are carried out routinely, on a large scale throughout industry, there remains a need for better separation methods, particularly in certain areas.
Chlorine ranks among the ten most important commodity chemicals produced worldwide. The total production of chlorine in the United States in 1991 was reported to be about 14 million tons, almost all of which was produced by the electrolysis of brine. The product of electrolysis is chlorine gas, contaminated with water, hydrogen, air, and other impurities. After the removal of water and other impurities, most chlorine is liquefied by compression and chilling, then sold. As with all compression/condensation processes, it is difficult to recover all the condensable chlorine gas without going to extreme conditions of temperature and pressure. It is not unusual, therefore, for the tail gas from the liquefaction process to contain as much as 40% chlorine by volume.
The presence of hydrogen in the gas stream is an added complication. When hydrogen is present in a gas stream with chlorine or with oxygen at hydrogen concentration less than about 4%, dependent upon pressure and temperature, usually the stream is non-explosive. However, as the hydrogen concentration increases above this lower explosive limit, the reaction on ignition becomes more violent and eventually may reach the detonation stage. To avoid this, the gas stream is routinely diluted with enough air or nitrogen to keep the hydrogen concentration below the 4% limit. Typically such additions are made after condensation steps, where removal of condensable components leaves a higher concentration of hydrogen in the vent stream.
For the past forty years, tail gas has been treated by absorption in carbon tetrachloride. Tail gas from chlorine liquefaction, and other waste streams ("sniff gas") from the plant, are supplied to the carbon tetrachloride absorber under pressure. Chlorine-free (.about.1 ppm) gas is vented to the atmosphere. The chlorine-rich carbon tetrachloride is fed to a stripper, where chlorine is desorbed and sent to the liquefaction system. The stripped solvent is pumped back to the absorption tower. Approximately 30 lb of carbon tetrachloride per ton of recovered chlorine are lost in this process. It is estimated that 9 million lb of carbon tetrachloride are emitted annually by chlorine liquefaction tail-gas treatment plants. Additional emissions result from similar chlorine absorption processes used in the paper, textile, and polyvinyl chloride industries. Because of the high ozone-depletion potential of carbon tetrachloride, the U.S. Environmental Protection Agency has mandated that these emissions be eliminated, and carbon tetrachloride production has ceased. There is an urgent need, therefore, for alternative treatment technology.
Another source of chlorine-laden gas streams is metal production by electrolysis of the respective molten chlorides, for example, magnesium, calcium, beryllium, and sodium chloride. In all cases, chlorine-containing gas is liberated at the cell anodes; this gas may typically contain as much as 90% chlorine. Other processes that require removal or recovery of chlorine from gas streams include, but are not limited to, production of chlorinated chemicals, bleaching, refrigeration and heat transfer fluids, chlorine transfer and clean-up operations, ore beneficiation, and wastewater treatment.
U.S. Pat. No. 5,538,535, co-owned with the present application, describes a membrane process for separating chlorine from chlorine-containing gas streams.